Leading-edge rain erosion poses a significant challenge for the wind turbine industry due to its detrimental effects on structural integrity and annual energy production. Developing effective mitigation strategies requires understanding the precipitation conditions driving erosion. The influence of the rain droplet diameter on both the formation of erosion damage and erosion mitigation strategies has yet to be sufficiently understood. This study proposes an enhanced damage model based on the impingement metric as used in the state of the art but improved by including important and thus far neglected physical mechanisms such as the recently described droplet slowdown and deformation effect. Several drop-size-dependent effects are identified within the damage model. Subsequently, their significance for leading-edge erosion is established for the International Energy Agency (IEA) 15 MW reference wind turbine, a site in the Netherlands and a commercial leading-edge coating. Thereafter, the influence of the drop-size effects on the viability of the erosion-safe mode (ESM) is investigated. The outcome is that drop-size effects strongly impact the erosion process and should not be neglected during modeling. Large droplets are considerably more damaging than small droplets, even when normalized for water volume. This directly influences the parameter space of erosion, such as the relevant droplet diameter range that should be studied. The drop-size effects shift damage production to higher rain intensities. Roughly half of the erosion damage is produced by only 10 % of rain events. When drop-size effects are excluded, this value shifts to more than 20 %. Regarding the ESM, we found that it can be utilized up to twice as efficiently when drop-size effects are adequately modeled. The findings highlight the criticality of drop-size effects in rain erosion modeling for wind turbine blades, impacting lifetime predictions, ESM viability and the parameter space of leading-edge erosion. This paper also provides a formal derivation of impingement and describes a method for finding optimal ESM strategies.@en

The erosion-safe mode (ESM) is a novel mitigation strategy that reduces rainfall-induced erosion damage by lowering the tip-speed of the turbine during precipitation events. The ESM requires accurate information about future expected rainfall for its control. In current research, it is debated what method or source should be used to this end. This study explores the effectiveness of driving the ESM using a state-of-the-art weather-radar-based probabilistic rainfall nowcast provided by the Royal Netherlands Meteorological Institute (KNMI). The performance of the nowcast is assessed for various lead times with an impingement-based damage model for three sample sites in the Netherlands and for two distinct ESM strategies. The results show that the quality of the nowcast degrades with increasing lead times, where the 5- and 15-minute lead times exhibit sufficiently good accuracy and response time for adjusting turbine speeds. Overall, the results highlight that the probabilistic information in the nowcast can be employed to improve the efficiency and viability of the ESM.

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Leading-edge rain erosion is a severe problem in the wind energy community since it leads to blade damage and a reduction in annual energy production by up to a few percent. The impact speed of rain droplets is a key driver of the erosion rate; therefore, its precise computation is essential. This study investigates the aerodynamic interaction of rain droplets and wind turbine blades. Based on findings from the literature and an analysis of the relevant parameter space, it is found that the aerodynamic interaction leads to a reduction in the impact speed. Additionally, the rain droplets deform and break up as they approach the wind turbine blade. An existing Lagrangian particle model, developed for research in aircraft icing, is adapted, extended, and validated for leading-edge rain erosion to study the process in more detail. Results show that the droplet slowdown reduces predicted damage toward the tip of the blade by over 50 %. The model indicates that the aerodynamic blade interaction affects small droplets significantly more than large droplets. Due to this drop-size dependency, the damage accumulation is shifted toward higher-rain-intensity events. Additionally, the droplet impact speed is sensitive to the aerodynamic nose radius of the airfoil. Due to this sensitivity and its drop-size dependency, the slowdown effect provides interesting levers for erosion mitigation via blade design or operational adjustments. To conclude, the aerodynamic interaction between droplet and blade is non-negligible and needs to be taken into account in erosion lifetime models. @en

Many wind turbines experience leading-edge erosion on their blades due to rain and hail impacting at speeds of up to 100 m/s. The impact speed is driven predominantly by the blade tip-speed, which is expected to grow in future turbine generations as they become larger. Erosion can remove substantial amounts of material from the blades. Eventually, the damage can reach deep into the structural layers of the blade, where it then starts to jeopardize its structural integrity. The associated roughening of the blade is accompanied by losses in the annual energy production (AEP). These are estimated to be up to several percent, depending on the severity of the erosion damage. While some leading-edge protection systems have been developed, no satisfying solution has been found, and the mechanisms that lead to erosion have yet to be fully understood. The aim of this thesis is to enhance the understanding of the physical mechanisms that promote erosion, understand which site conditions contribute to erosion and apply the gained insight in the erosion-safe mode.

This thesis starts in Chapter 2 by analyzing the impact of erosion on the AEP loss by using reduced-order modeling and subsequently compares it with the erosion-safe mode (ESM). The ESM is an alternative operational erosion mitigation strategy that aims to mitigate erosion by reducing the tip-speed of the turbine during precipitation events. It is shown that, depending on the mean wind speed and frequency of damaging rain at the site, the erosion-safe mode can lead to a lower AEP loss in comparison to a mildly eroded blade or a blade that was fitted with a leading-edge protection solution that leads to similar flow disturbance. However, it still needs to be sufficiently understood what rain is damaging and what other site conditions might promote erosion.

A step toward resolving this knowledge gap is taken in Chapter 3 by investigating the behavior of rain droplets before impact with the blade. Contrary to prior state-of-the-art, it is shown that droplets deform and break up near an incoming wind turbine blade. This finding contradicts the current approach in erosion research of modeling rain droplets as circular. It is shown that deformation reduces the impact velocity of rain droplets with the blade. This effect depends on the diameter of the rain droplets and can be in the order of 10 m/s. Small droplets experience significantly more slowdown than larger rain droplets. This reduction highly influences the formation of erosion damage since the main driver for erosion is the impact velocity. As droplet deformation and slowdown depend on the rain droplet diameter, the described effect can be termed drop-size-dependent effect.

Chapter 4 continues the investigation of drop-size-dependent effects in leading-edge erosion. An advanced erosion damage model is built that includes several drop-size-dependent effects. It is shown that the significant drop-size-effects all suggest that the erosiveness of rain droplets increases with increasing droplet diameter. This is found to be true on a per-drop basis but also when normalizing for droplet size. Therefore, selecting an appropriate droplet diameter for experiments and numerical studies is essential since not all droplet diameters contribute equally toward forming erosion damage. Drop-size effects have substantial implications for the ESM, as increasing rain intensities shift the composition of precipitation from primarily small droplets to a composition dominated by larger ones. For an equal rain column, high-intensity precipitation events are, hence, more erosive. It is found that, for a coastal site in the Netherlands, 50 % of the erosion damage is produced by the 10 % highest-rain intensity events. Thus, in ESM operation, it is advantageous to reduce the tip-speed mainly during high-intensity precipitation events to maximize lifetime and minimize AEP loss. However, a precise relation between precipitation intensity and tip-speed that optimizes this objective is not yet known in leading-edge erosion research. A novel semi-analytical approach is devised to bridge this gap, taking into account site conditions, turbine type, and drop-size effects. With this approach, it is possible to extend the erosion lifetime of a contemporary blade by a factor of 13 for a moderate AEP loss of 1 %.

A critical component for the successful utilization of the ESM is the accurate forecasting of precipitation events minutes to hours ahead. However, the best approach for obtaining this information is still debated. For the first time, Chapter 5 benchmarks a state-of-the-art weather-radar-based probabilistic rainfall nowcast product by the Royal Netherlands Meteorological Institute (KNMI). The performance of the nowcast is assessed for various lead times for three sample sites in the Netherlands and for two distinct ESM strategies. The results show that the quality of the nowcast degrades with increasing lead times. The 5- and 15-minute lead times exhibit sufficiently good accuracy and response time for the successful utilization of the ESM. Across the sites, for a large 15 MW turbine, a lifetime extension of factor five can be achieved for an AEP loss of about 1 %.

To summarize, this thesis introduced the highly significant effect of droplet slowdown and deformation occurring in the vicinity of wind turbine blades. It investigated drop-size-dependent effects and established their significance for ESM operation. It provided new theoretical insights into the ESM and used these to devise a method for finding optimal ESM strategies that exploit drop-size effects. Finally, it benchmarked the devised strategies using a state-of-the-art (operational) nowcasting product and showed that the ESM could already be a viable erosion-mitigation strategy.@en

For offshore wind turbines, Leading-Edge Erosion (LEE) due to rain is posing a serious risk to structural integrity and can lead to a performance loss of the order of a few percent of the Annual Energy Production (AEP). A proposed mitigation strategy is the so-called Erosion-Safe Mode (ESM). In this work, the AEP losses caused by LEE or by operating in the ESM are compared for two reference turbines, i.e. the IEA 15MW and the NREL 5MW turbines. For both turbines, the performance is evaluated in uniform and sheared inflow conditions. The effects of erosion are modeled by creating clean and rough airfoil polars in XFOIL. It is assumed that erosion occurs once a critical blade element section speed is exceeded. Power curves for LEE and ESM are calculated by using the free-wake vortex method CACTUS. Results show that LEE negatively affects the power production below rated capacity, while operating in ESM predominantly sheds performance at rated power of the turbine. This study, therefore, shows that a break-even point for the ESM exists. The AEP loss due to erosion can be successfully mitigated with the ESM at sites with low mean wind speed, however, at sites with higher mean wind speed, operation with erosion leads to a lower AEP loss. The break-even point shows little sensitivity to the blade design and to mean shear variations, but strongly depends on the frequency ESM needs to be applied. The latter is driven by the predicted amount of damaging rain events. In conclusion, erosion-optimal operation is strongly governed by the site characteristics and much less by turbine design, and the viability of an ESM strategy can be significantly expanded by a better understanding of blade damage mechanisms and improved forecasting of the related weather events.

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